Many approaches for waste disposal are currently available. For example, sanitary landfills formed by filling a land area with successive layers of solid waste and layers of earth or soil are well known. Unfortunately, such landfills have the potential for producing large amounts of a hazardous, explosive gas (methane), which may migrate to buildings or structures several hundred feet from the landfill if not removed from the landfill. Further, the natural precipitation draining out of the landfill may carry toxic, polluted water to contaminate underground water supplies, surface streams, and wells. Due to the very slow stabilization of waste, a landfill may not be used for other purposes for long periods of time and, thus, particularly near metropolitan areas, represents a large waste of land.
Other approaches utilize anaerobic digestion for stabilization and conversion of organic wastes to methane and compost. In general, such anaerobic designs convert a large fraction of organic matter (>50%) to methane and carbon dioxide without the need for hydrolysis as a pretreatment step or extensive external energy requirements to remove water (i.e., with thermal processes) or pretreatment and product recovery (i.e., with bioethanol).
One such anaerobic design utilizes a leachbed anaerobic composting process (hereinafter the SEBAC system or process) for the anaerobic digestion of high-solids organic feedstocks, as disclosed in U.S. Pat. No. 5,269,634. In this process, coarsely shredded feedstock is placed into a bioreactor and leachate from a mature bioreactor recycled between the mature bioreactor and the newly loaded bioreactor to provide moisture, inoculum, nutrients, and buffer necessary to start up the newly loaded bioreactor. Such bioreactors, however, require a great deal of volume due to the bulk density of organic solids introduced into the bioreactors. Further, for efficient composting, such bioreactors must be maintained at thermophilic levels (about 55° C.); which require a great deal of expensive energy input.
To address these deficiencies, improved SEBAC systems have been proposed (see Chynoweth, D. et al., “Anaerobic Digestion for Reduction and Stabilization of Organic Solid Wastes During Space Missions: Laboratory Studies,” 32 Internat. Conf. Environ. Systems (ICES), paper 2002-01–2351 (Jul. 15, 2002)) in which SEBAC bioreactors are flooded and leachate is forcibly pumped between the flooded bioreactors. Unfortunately, within the bioreactors of such flooded SEBAC systems, generated biogas often becomes entrapped in the densified solid waste and displaces a significant volume of leachate into the biogas line. This causes a loss of liquids within the enclosed system, which is a concern because it affects proper function of SEBAC systems.
Further, during flooded SEBAC operation when introducing newly filled bioreactor(s), the flow of leachate through the leachbed of mature bioreactor(s) is impeded or may even cease, in spite of the use of positive displacement pumps. Specifically, when floatation of waste residue comes into contact against an upper screen of a newly filled bioreactor, an increase in pressure drop (hydraulic pressure) across the reactor bed is observed. Without the flow of leachate through the flooded SEBAC system, liquid flow through the reactor may cease and thus hinder anaerobic digestion of organic solid wastes.
Accordingly, an improved flooded SEBAC system and process for the efficient anaerobic conversion of organic wastes is needed to address the deficiencies noted above, namely displaced leachate as a result of biogas entrapment and impeded leachate flow, both of which result in increased hydraulic (leachate) pressure in bioreactors and hindrance of anaerobic digestion of organic wastes.